A density-functional-theory-based relativistic scattering formalism is used to study charge transport through thin Pt films with room-temperature lattice disorder. A Fuchs-Sondheimer specularity coefficient $p\sim 0.5$ is needed to describe the suppression of the charge current at the surface even in the absence of surface roughness. The charge current drives a spin Hall current perpendicular to the surface. Analyzing the latter with a model that is universally used to interpret the spin Hall effect in thin films and layered materials, we are unable to recover values of the spin-flip diffusion length $l_{\mathrm{sf}}$ and spin Hall angle ${\mathrm{\Theta }}_{\mathrm{sH}}$ that we obtain for bulk Pt using the same approximations. We trace this to the boundary conditions used and develop a generalized model that takes surface effects into account. A reduced value of ${\mathrm{\Theta }}_{\mathrm{sH}}$ at the surface is then found to describe the first-principles transport results extremely well. The in-plane spin Hall effect is substantially enhanced at the surface.

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• Quantum transport
• Spin Hall effect
• Ultrathin films
• Density functional theory
• Methods in transport